Here is the opening of an essay of mine at the Library of Economics and Liberty  on getting the most out of life and the concept of opportunity cost:
One of the challenges of being an economist is explaining what you do
for a living. People understand that one of the things a professor of
economics does is teach economics. But what is that, exactly? Most
presume it has something to do with investing and financial management.
When I once told my seatmate on an airline flight that I was an
economist, she said, what a shame, my husband loves the stock market.
Hmm. I didn’t tell her that other than the advantages of investing in
indexed mutual funds, I know next to nothing about the stock market.
My seatmate might have profited from reading Alfred Marshall  who called economics "the study of mankind in the ordinary business of life." This was the enterprise of Marshall and Adam Smith  and Friedrich Hayek  and Milton Friedman : they tried to understand what people do and the implications of their behavior for the society at large.
But my favorite definition of economics is a variant of Marshall’s. It
comes from a student who heard it from another teacher of hers:
economics is the study of how to get the most out of life. I like this
because it strikes at the true heart of economics—the choices we make,
given that we can’t have everything we want. Economics
is the study of infinite wants and finite means, the study of
constrained choices. This is true for individuals and governments,
families and nations. Thomas Sowell said it best: no solutions, only
tradeoffs. To get the most out of life, to think like an economist, you
have to be know what you’re giving up in order to get something else.